The present invention relates to a method of forming a buried interconnecting wire by filling a recessed portion formed in an insulating film, such as a groove, with a metal for interconnections in the process of manufacturing a semiconductor integrated circuit device or the like. In particular, it relates to technology for preventing the metal for interconnections from being oxidized or diffused into the insulating film.
At present, an aluminum alloy is used predominantly as a metal for interconnections in a semiconductor integrated circuit device or the like. On the other hand, copper or a copper alloy is receiving attention as a promising replacement for the aluminum alloy to be used in the next generation because of its lower resistivity and higher immunity to electromigration.
The largest problems presented by copper interconnections composed of copper or a copper alloy are the oxidation of the copper interconnections, the diffusion of copper from the copper interconnections into an insulating film, and poor processibility of a copper film, which remain to be solved before the copper interconnections are used in practice. In particular, the copper or copper alloy composing the copper interconnections is easily oxidized by and diffused into a SiO.sub.2 film used for an interlayer insulating film, which may adversely affect a device such as a transistor formed under the interlayer insulating film. To solve the problems, there has been proposed the formation of various barrier layers between the copper interconnections and the interlayer insulating film.
For example, Japanese Laid-Open Patent Publication HEI 02-240920 proposes a method of forming a barrier layer composed of a TiN film by performing N.sub.2 annealing with respect to a Cu--Ti alloy at a temperature of 800.degree. C. to prevent the oxidation and diffusion of copper.
On the other hand, Japanese Laid-Open Patent Publication HEI 06-275623 and "Diffusion Barrier Properties of Transition Metals and Their Nitrides for Cu Interconnections (T. Nakao et. al, VMIC (1994))" propose a method of forming a barrier layer composed of a tungsten nitride film by nitriding a W film by using a plasma in accordance with an ECR plasma method.
In the case of forming multi-layer metal interconnections by using the copper interconnections composed of copper or a copper alloy, the temperature of a heat treatment for a barrier layer should be 600.degree. C. or lower to prevent the oxidation and diffusion of copper in the underlying copper interconnections. However, a TiN film as formed by N.sub.2 annealing in accordance with the foregoing method cannot be implemented at a temperature of 600.degree. C. or lower. If the TiN film is formed by N.sub.2 annealing at a temperature of about 800.degree. C., on the other hand, the copper in the underlying copper interconnections may be oxidized and diffused disadvantageously. What results is the problem that the use of the copper interconnections is incompatible with the formation of the barrier layer by N.sub.2 annealing.
In the case of the latter method, the use of copper interconnections is compatible with the formation of the barrier layer by plasma nitriding, since the latter method allows the formation of a uniform barrier layer at a low temperature and hence is free from the problems of the oxidation and diffusion of the copper in the underlying copper interconnections. Unlike aluminum interconnections, it is extremely difficult to form copper interconnections from a copper film by performing dry etching with respect thereto, since a halogen compound is non-volatile.
To overcome the difficulty, there has been proposed a method of forming buried interconnections from copper by forming grooves in a region of an insulating film in which the interconnections are to be formed, depositing copper over the entire surface to form a copper film so that the copper is filled in the grooves, and removing portions of the copper film located outside the grooves.
In the case of forming the copper interconnections in accordance with such a method of forming buried interconnections, the barrier layer should be formed not only on the bottom of the grooves but also on the sidewalls thereof. In Japanese Laid-Open Patent Publication HEI 06-275623, as described above, the barrier layer composed of the metal nitride film is formed by depositing the metal on the bottom and sidewalls of the grooves to form the metal film and then plasma-nitriding the metal film under a pressure of 1 mTorr in accordance with the ECR plasma method.
However, the conventional method of forming the barrier layer composed of a metal nitride film by performing a plasma-nitriding process at 1 mTorr has such problems as illustrated in FIGS. 6(a) and 6(b). FIG. 6(a) illustrates the process steps of depositing a silicon dioxide film 12 on a silicon substrate 11, forming a groove 13 in the silicon dioxide film 12, depositing a tungsten film 15 over the entire surface of the silicon dioxide film 12, and then forming a tungsten nitride film 17 on the surface of the tungsten film 15 by a plasma-nitriding method. In this case, the mean free path of nitrogen ions at 1 mTorr is 10 cm or more, which is much larger than the sheath length (about 3 mm) of a sheath region between a plasma generation region and the silicon substrate, so that the nitrogen ions have an extremely low probability of colliding with nitrogen molecules in the sheath region. Accordingly, nitrogen ions 16 are incident upon the silicon substrate 11 in a direction substantially perpendicular thereto, as shown in FIG. 6(a). As a result, the nitrogen ions 16 seldom reach these portions of the tungsten film 15 covering the sidewalls of the groove 13, where a nitriding reaction does not proceed, so that the tungsten nitride film 17 is not formed on the sidewalls of the groove 13.
If a copper film 18 is deposited over the entire surface of the substrate with the tungsten nitride film 17 being not formed on the portions of the tungsten film 15 covering the sidewalls of the groove 13, copper contained in the copper film 18 is diffused into the silicon dioxide film 12 through the portions of the tungsten film 15 covering the sidewalls of the groove 13 because of unsatisfactory barrier property of the tungsten film 15, which adversely affects a device formed on the silicon substrate 11.
Although the foregoing process of forming the metal film by depositing a high-melting-point metal on the bottom and sidewalls of the groove is preferably performed by CVD which provides excellent coverage over the bottom and sidewalls of the groove, the following problem arises during the process: If the crystal growth of the high-melting-point metal is promoted to deposit a metal film having a low resistivity, undulations are formed on the surface of the metal film so that the plasma-nitriding process proceeds on some portions of the metal film, while stagnating on others, due to the presence of the undulations. Hence, a barrier layer composed of an equally nitrided metal nitride film cannot be obtained.